1. Technical Field
The present invention relates to rotors for rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of Related Art
There are known IPM (Interior Permanent Magnet) motors that have permanent magnets embedded in a rotor core. The IPM motors can use both reluctance torque and magnetic toque, thereby achieving high efficiency. Therefore, the IPM motors are particularly suitable for use in hybrid and electric vehicles.
An IPM motor generally includes a stator and a rotor that is disposed in radial opposition to the stator. The rotor includes a rotor core and a plurality of permanent magnets. The rotor core has a plurality of pairs of magnet-receiving holes formed therein. Each pair of the magnet-receiving holes is arranged in a substantially V-shape that opens toward the stator side. Each of the permanent magnets is received in a corresponding one of the magnet-receiving holes of the rotor core. Further, for each pair of the magnet-receiving holes, the two corresponding permanent magnets which are respectively received in the two magnet-receiving holes of the pair are arranged so as to together form one magnetic pole of the rotor. In addition, for each pair of the magnet-receiving holes, there is formed a corresponding center bridge that extends in a radial direction of the rotor core between the two magnet-receiving holes of the pair to separate them from each other.
Japanese Unexamined Patent Application Publication No. 2000-278896 (to be simply referred to as Patent Document 1 hereinafter) discloses a technique of suitably designing magnetic flux barriers at opposite ends of each magnetic pole of the rotor, thereby reducing cogging torque of the motor.
However, with the technique disclosed in Patent Document 1, the q-axis inductance may be lowered and magnetic flux leakage via the corresponding center bridge may be increased.
Japanese Unexamined Patent Application Publication No. 2011-211860 (to be simply referred to as Patent Document 2 hereinafter) discloses a technique of providing magnetic flux barriers on the radially inside (i.e., the side closer to a longitudinal axis of the rotor core) of each magnetic pole of the rotor, thereby reducing magnetic flux leakage in a radially inward direction.
However, the technique disclosed in Patent Document 2 cannot be applied to rotors where there is insufficient space for providing the magnetic flux barriers in the vicinity of a radially inner periphery of the rotor core.